The invention relates to a device for fastening a tool member on a drive shaft of a motorically driven power tool, comprising a support plate for the tool member, wherein the drive shaft projects through the support plate, the tool member, a rider plate, and a clamping element. The drive shaft supports a clamping device that, by means of the clamping element, fixes the tool member on the support plate.
A device for fastening a tool member on a tool head is disclosed in U.S. Pat. No. 6,032,369. The support plate comprises a hub that projects through a central opening of the tool member. The support plate and the tool member are penetrated by the output shaft of a gearbox wherein a pressure plate, a rider plate, and a clamping device are arranged on the output shaft. The clamping device is designed as a clamping nut that is screwed onto a threaded section of the end of the output shaft. The clamping nut acts on a fastening rim of the rider plate which is supported on the pressure plate. The pressure plate is contacting the tool member that is arranged on the support plate.
When tightening the clamping nut, the clamping force is transmitted by the fastening rim of the rider plate onto the pressure plate wherein the fastening rim is secured by being clamping between the clamping nut and the pressure plate. When the rider plate touches the ground during working with the tool head, a braking moment is acting on the rider plate. Since the clamping nut is immediately contacting the fastening rim of the rider plate, torque is acting on the clamping nut and can cause loosening of the clamping nut on the thread of the output shaft. The torque can also cause an excessively strong tightening of the clamping device which makes it difficult to release the arrangement and/or can cause overload of the arrangement.
In order to counteract this loosening of the clamping nut or an overload of the arrangement, it can be provided in case of clockwise rotating drive shafts to provide a left-hand thread and in case of counterclockwise rotating drive shafts to provide a right-hand thread. In particular in case of power tools that are driven in different rotational directions during operation, it is thus only possible to provide a securing action in one rotational direction.
The invention has the object to provide a device for fastening a tool member on a rotating output shaft that, independent of the rotational direction of the output shaft, prevents loosening of the clamping device when the rider plate comes into contact with the ground.